Passive-Wireless SAW (PWSAW) sensors have been produced for remote sensing of temperature and strain, and by using specially designed packages, the strain sensors have been adapted for use as torque or pressure sensors. Also the PWSAW devices have been used as short range radar transponders that facilitate the measurement of distance, speed and revolutions per minute (RPM).
The PWSAW sensor devices use the properties of a piezoelectric substrate material to convert a received RF interrogation pulse into an acoustic wave that travels on the surface of the substrate; thus, the name Surface Acoustic Wave (SAW). As shown in FIG. 1, a PWSAW device 8 comprises a Radio Frequency (RF) transceiver 10 for generating an interrogation pulse 12 that is transmitted from an antenna 14 through the air and received by an antenna 16 attached to the PWSAW device 8.
The received RF signal is then applied to an Interdigital Transducer (IDT) 22 consisting of metal electrodes or “fingers” 26 etched on the surface 28A of the piezoelectric substrate 28.
The IDT 22 generates a SAW pulse 30 that propagates to a series of reflector arrays (RAs) 32, which each RA also comprising a plurality of metal electrodes or “fingers” 34. Each finger 34 in an RA reflects a small portion of the remaining energy in the incident SAW wave. However, these “sub-echoes” from each finger are designed to be coherent with each other, so their sum can be treated as a single echo from the RA (the sum echo referred to herein as a reflected SAW or echo).
When these RA echoes reach the IDT 22, they are converted back to electrical signals (an electrical echo pulse) and immediately radiate from the attached antenna 16 back to the transceiver 10 where they are processed to produce sensor data.
The transceiver 10 is controlled to operate in a transmitting mode to transmit the interrogation pulse 12 during a first interval and later in a receiving mode to receive the electrical echo pulse during a second interval.
FIG. 2 illustrates propagating SAWs 44 on the substrate 28.
Stress in the crystalline structure of the piezoelectric substrate changes when its temperature changes or when mechanical forces are applied to it. This stress changes the propagation velocity (Vp) of the SAW wave from the value of the propagation velocity in the absence of any imposed stresses. With a change in the propagation velocity the differential time delay and the differential phase shift are similarly changed. Here time delay refers to the time difference between receipt of an echo from a first RA and an echo from an adjacent second RA. Similarly, the phase shift refers to the phase shift between these echoes received from the first and second RAs.
The round-trip propagation time for a SAW from a time when it is launched from the IDT, reflected from the ith RA, and returned to the IDT is determined by Vp and by the distance to the ith RA (di). This round-trip propagation time is given by:τi=(2*di)/Vp  [1a]
The value τi may also be referred to herein as a delay time representing a time interval between launching of an incident SAW from the IDT and returning of the echo or reflected SAW from the ith RA back to the IDT.
For all cases of practical interest the distance traveled by the SAW signal on the substrate is two-way; therefore, it is useful to define a two-way propagation velocity, Vp2, to avoid the need to double distances when performing calculations. By dividing the numerator and denominator in Equation [1a] by a factor of two, we getτi=di/Vp2  [1b]where Vp2 is half the value of Vp.
For a Y-cut lithium-niobate substrate at 25° C., Vp is 3488 m/sec, and Vp2 is 1744 m/sec or 1.744 μm/nsec, as expressed in units that are more useful for design analysis.
Since both the RF signal and the resulting SAW are sinusoidal waves, the absolute phases of the RA echoes (ϕi) are determined by the absolute time delays of the RA echoes (τi) as set forth in Equations [3a] and [3b] below.
The delay τi will be many times or multiples of the period (P) of the sinusoidal carrier signal plus some fraction (ΔP).
Note that P is 1/fc, where fc is the carrier frequency (e.g., fc=430 MHz or fc=900 MHz) of both the RF or SAW wave.
The echo signal received at the RF transceiver exhibits a phase consistent with this fractional part, ΔP, and because each cycle of a sine wave is indistinguishable from previous or later cycles, the integer periods of P are ambiguous and can be removed by using a modulo function.
But the remaining fractional part, ΔP, is very significant and in fact forms the basis of the sensor measurements.
The phase of the echo signal is given byϕi=2πτi/P or ϕi=2πfcτi  [2]noting that τi is actually “τi modulo P”, which removes ambiguous integer multiples of P.
From Equation [1b] above, and considering thermal effects on the distance-related parameters, the propagation velocity is more accurately given as a function of the temperature T according to the equationτi(T)=(di/VP2)*(1+((T−25)*TCD))  [3a]where di, VP2, and VP are only defined at T=25° C., which therefore requires the use of the thermal coefficient of delay parameter (TCD) in the above equation for other values of “T”.
And substituting equation [3a] into equation [2]ϕi(T)=2πfcτi(T)=2πfc(di/VP2)*(1+((T−25)*TCD))  [3b]
Equation [3b] illustrates the role of Vp2 and di in determining the echo phase.
Recall that the effect of temperature or strain on the substrate is to change Vp, which according to equation [1a], inversely changes the time delay (τi). The change in delay is nearly linear with temperature and is called the Thermal Coefficient of Delay (TCD), which is well documented for the various materials used in SAW fabrication and appears in Equations [3a] and [3b] above. For instance, for lithium-niobate the TCD is 94 parts-per-million/° C. (94 ppm/° C.). By noting that di and Vp2 are only defined at 25° C. we can now see how the sensor works by making τi and ϕi dependent on the temperature (T) of the substrate.
Note that the distances di or differential distances (dj−di), are actual physical distances when measured at 25° C., therefore Equations [1a], [1b], [3a], and [3b] are correct for VP or VP2 which are also defined only at 25° C. But, while VP, τi and ϕi change proportionately with temperature according to the coefficient, TCD, the distances, di or (di−dj), do not change proportionately. TCD therefore represents a combined effect, partly caused by a change in physical distance (i.e., expansion or contraction) and partly by a change in the stiffness of the crystalline substrate. TCD is several times larger than the coefficient of thermal expansion (CTE). Since it is not strictly correct to scale the separation distances by TCD, in this document all references to “di” should be interpreted as a distance at 25° C.
However, using the concept of “virtual distance” (where di values are scaled by TCD) is often useful and this can be implemented in certain embodiments. Furthermore, both the expansion and stiffness changes create a greater τi at higher temperature and a smaller τi at lower temperatures, so one coefficient is sufficient to reflect their combined result.
The time delays and phase shifting that make sensing possible are accomplished while the signal is in its acoustic form (i.e., a SAW), however, changes in these parameters are also observed directly in the RF wave received back at the interrogating transceiver 10 of FIG. 1.
The sensing capabilities of a PWSAW are based on differential delays of the echoes from the various RAs (in particular the echoes from adjacent RAs), rather than the absolute delay from any one RA or from a group of RAs. If this were not the case, the sensors could be made with a single RA and the time delay would be measured as precisely as possible in order to detect changes in VP. However, this technique does not yield the precision that can be achieved with multiple RAs.
Returning to FIG. 1, the PWSAW sensor 8 comprises the various metal electrodes (IDT and RAs) that are deposited on a small piezoelectric substrate using standard semiconductor fabrication techniques. In one application the substrate is about 12 mm×2 mm. As described above, these metal electrodes launch, reflect and receive the surface acoustic waves (SAWS).
Note that the creation of a SAW wave by the IDT and the reflection back from the RA structures on the device are totally passive processes similar to a mirror reflecting light, except that the signal is at RF frequencies rather than visible light frequencies. Due to properties of the piezoelectric substrate, conditions of interest in the environment at the sensor create predictable variations in the echo signal produced by a PWSAW device. Thus allowing the SAW device to function as a sensor.
The SAW wave velocity, Vp, on the piezoelectric substrate is about 1/100,000th the speed of the RF wave traveling in free space. So the SAW device adds a significant delay (equivalent to a distance of several kilometers) to the signal before retransmitting the RF echo signal. This echo delay gives the RF transceiver 10 of FIG. 1 time to complete its transmission and switch to receive mode before echoes arrive from the SAW sensor.
Also, due to this relatively low velocity and the very high ultrasonic frequency (anywhere from 200 MHz to 2000 MHz can be used for sensing), the SAW wave has a wavelength of only a few microns (e.g., 3.8 μm for a 915 MHz wave on lithium-niobate) which allows it to interact with very small features on the substrate.
When several SAW sensors must be operated simultaneously, the sensors are encoded with various time delays that enable the receiving system (i.e., the receive and attendant processing components) to isolate and identify data from each individual sensor.